Computer Data Storage
Computer data storage which is co-located with a host, server, or an individual computer is placed in a section of computer memory designated primary storage. This primary storage provides the computer with the capability to process program instructions, to store input data, to retain statements from programs undergoing processing, to retain and use data produced by programs undergoing processing, and to format data in preparation for output.
Initially data which goes into the computer is held in primary storage until the data is used during the processing of program instructions. During processing of the data by the computer, the primary storage is used to store the intermediate and final results of all arithmetic and logical operations. Intermediate results are temporarily retained in storage areas which are often referred to as working storage. Working storage areas are specifically set aside for the temporary storage of data while a program is being run on one or more computers. While the computer is running the program and using the data, the computer may be acting as if all program instructions and needed data are resident in its main memory; however, the program instructions and data may actually be located miles away at another location and downloaded into the computer as if resident in the computer's primary storage. Such storage configuration is called virtual storage. The final results from the running of a computer program are transferred to storage areas that have been designated by the program instructions. These final results will remain at the designated storage areas until output operations begin.
In addition to storing the data involved in processing, the primary storage of a computer must retain the program instructions used for processing data. As computer applications have become more sophisticated, the size of computer programs has grown exponentially. Such exponential growth in the size of computer programs has necessitated the use of virtual storage of computer programs or data in locations away from the host or server computer to enable sufficient space in primary storage to run other large computer programs.
For example, a large international bank may have a designated time each day to consolidate all of the transactions from its branches around the world. Once this daily consolidation has taken place, the massive amount of consolidated data must be stored somewhere for later use in putting together periodic account statements and to provide the specialized reports necessary to meet regulatory and tax requirements. Such specialized reports could not be run unless the bank's computer had access to large virtual data storage capacity.
Secondary or Back-up Data Storage
Because of the ever growing need for data storage capacity to service larger and larger computers which run larger and larger programs--all of which produce massive amounts of data--the need has arisen for larger and more sophisticated secondary or back-up data storage systems. Secondary data storage provides a computer system with greater data processing potential because it expands the amount of available storage space to a computer or a computer network for holding data. The data contained in secondary or back-up data storage may be too large to be held in primary or working storage; however, to allow the program resident in the host or server computer network to run, data storage space must be located and made available to the host or server computer as quickly as possible for data writing and data reading operations. By providing a host or server computer network with large amounts of secondary or back-up data storage capacity, the operational capabilities of individual computers or a computer network can be maximized.
Removable Media
One of the most used forms of secondary or back-up data storage device hardware is a removable media data storage unit. While slower and possibly more cumbersome than other types of secondary data storage, removable media data storage units such as tape cartridges remain a viable and frequently used data storage option because of its low cost and acceptance around the world.
Because of the way data is actually placed on one or more individual removable media data storage units, a removable media data storage system includes a controller to govern its operation. The removable media data storage system controller is used to divide computer programs and associated data into sections to guide the data being written onto one or more removable media data storage units from the host or server computer network to an individual removable media data storage unit. Such guidance from the removable media data storage system controller as to the path by which data is written on one or more individual removable media data storage units should be invisible to the host or server network. Rather, all the host or server computer network needs to retain is a "volume number" designating the location of the stored data. The term "volume number" actually comes from early magnetic tape data storage systems where a volume number was the identification number of a single reel of magnetic tape. Presently, a "volume number" may identify a single removable media data storage unit or a group of removable media data storage units where data is written to or read from.
When stored data is to be read back to one or more computers in a network of host or server computers, the removable media data storage system controller retrieves the stored data from the one or more removable media data storage units where it is actually stored and supplies the stored data to the host or server computer as if the data came from a single volume number location.
Data Storage on Multiple Removable Media Units
Because one of the problems in reading and writing data onto individual removable media data storage units is the lack of speed in the sequential access of data, the speed of either writing to or reading from secondary or back-up data storage on a removable media data storage unit has been greatly increased by the use of mass removable media data storage systems such as mass tape cartridge data storage systems which employ strips of magnetic tape in an ordered array. In these mass tape cartridge data storage systems, any data recorded in a tape cartridge can be accessed directly without having to search the entire contents of the tape cartridge.
In many removable media data storage systems, data is stored by a process known as striping. In striping a serial or linear sequence of groups or bytes of data is downloaded by parallel data groups into parallel groups of individual removable media data storage units. (In early magnetic tape data storage systems, bytes of data were written sequentially in a serial manner onto a single roll of magnetic tape. The single roll of magnetic tape then had to be either wound or unwound to gain access to the correct length of magnetic tape holding the needed data to read the needed data back into the computer.) Thus, if seven individual removable media data storage units are available for data storage, data can be written to storage on the removable media data storage unit or read from storage on the removable media data storage unit in groups of seven stripes.
Access to groups of individual removable media data storage units is typically accomplished by the use of robotic control or a robot. The removable media data storage system controller directs the robot to the group of individual removable media data storage units on which data is written to or read from. The robot then locates the appropriate group of individual removable media data storage units designated by the removable media data storage system controller for proper storage or retrieval of data. While the data may be written to or read from a group of individual removable media data storage units, the host or server network sees the data being written to or read from the removable media secondary or back-up data storage facility as being written to or read from a single virtual volume designated by single volume number or a single volume number including the location of a group of individual removable media data storage units. The formulation of the single virtual volume designation is accomplished by the removable media data storage system controller.
Commonly available removable media data storage systems or removable media data storage libraries include 15 removable media data storage units. This library of 15 removable media data storage units may be configured into two sets of seven removable media data storage units (with a spare in case a removable media data storage unit is out for service or cleaning). Sets of seven removable media data storage units are configured by the removable media data storage system controller to appear to a host or server in a computer network as a single volume number. Some manufacturers have found that as few as five removable media data storage units can be configured together so that a computer in a host or server network will still see the group of five removable media data storage units as a single volume number.
To increase secondary data storage capacity on removable media data storage systems, such as magnetic tape cartridge data storage system attempts have been made to serially connect together a string of mass magnetic tape cartridge data storage systems I, II, . . . , n as shown in FIG. 1. While additional secondary magnetic tape cartridge data storage capacity can be provided by this serial connection of magnetic tape cartridge data storage systems, the serial connection causes other problems to arise. Specifically, the serial connection S between the mass magnetic tape cartridge data storage systems limits access to other on-line magnetic tape cartridge data storage systems so that only one mass magnetic tape cartridge data storage system can be written to or read from at anyone one time. Thus, while a write operation or a read operation is taking place on one mass magnetic tape cartridge data storage system on the serial connection S, the remainder of the mass magnetic tape cartridge data storage systems on the serial connection S are rendered unusable.
Other Secondary Data Storage Systems
In addition to magnetic tape cartridge data storage, there are other systems associated with the secondary storage of computer data. Such other systems include compact disks (often called CD-ROM's), magnetic disks, magnetic drums, data cells, optical memory data storage systems, magnetic bubble data storage systems, laser storage systems, holographic data storage systems, optical scanners, printers and storage medium changers. A group of these secondary data storage systems has become known in the industry as a Redundant Array of Independent Data Storage Devices or a RAID. A numeral suffix indicates the number of data storage devices available, e.g. RAID3. Each individual storage device within a RAID is identified by its own Logical Unit Number or LUN for address purposes.
Many individual secondary data storage systems have operational protocols that are significantly different than the operational protocols used to store data on other individual secondary data storage systems. For example, the output of a typical magnetic tape cartridge data storage controller will not permit writing data to or reading data from anything other than magnetic tape. Other types of more advanced secondary storage systems are able to write data into storage or read data from storage in any format and therefore are completely compatible with and will accept data passed through a system using another storage medium. Some in the industry have referred to a RAID or a grouping or an array of data storage devices with shared data storage capacity as a Storage Area Network.
Demands on Removable Media Secondary Storage Systems
In addition to providing a large amount of secondary data storage capacity on removable media data storage units, there are still other requirements that are placed on secondary data storage systems which use removable media data storage units. Such other requirements include: (a) back up secondary data storage capacity, (b) recreation of secondary data storage, and (c) fault tolerance or the ability to provide a location for data storage should one or more individual removable media data storage units become inoperable or unavailable when the data is to be written on or read from one or more removable media data storage units. One method of meeting the need for backup secondary storage capacity, the need to recreate secondary data storage and the need for fault tolerance is to provide a second, redundant, identical array of data storage capacity. The process of providing a second, redundant array of data storage capacity is known in the industry as "mirroring." To prevent unforseen occurrences such as power failures, fires, earthquakes, storms, or sabotage from interfering with the writing on and reading from removable media data storage unit, such mirrored removable media data storage units may be located thousands of miles apart.
Finally, there is the ever present demand for greater speed in both writing data to storage and reading data from removable media data storage units to meet the requirements of newer computers that can process data at ever increasing speeds. To meet the demand for greater speed in both writing data to and reading data from removable media data storage units, removable media data storage units must be both able to physically transfer data faster to and from removable media data storage units as well as provide increased availability of those removable media data storage units not in use.